Direct overwriting is desirable to provide improved performance by not requiring a revolution of the disk for erasing data. It also provides better control of mark lengths relative to space lengths and permits higher linear densities through partial response signalling.
To achieve direct overwriting in magneto-optic recording, the magnetic field of a magnetic head must be switched faster than the encoded bit rate. In systems where the magnetic head must be about a millimeter away from the active layer, the large volume of magnetic field needed for writing under such conditions requires high power to switch the field rapidly.
U.S. Pat. No. 4,872,078 issued Oct. 3, 1989, assigned to the assignee of the present invention, describes a method whereby direct overwrite is achieved by a resonant circuit approach. Data may be directly overwritten by locally heating the active layer of the magneto-optic medium with a short pulse of laser light in the presence of a periodically alternating magnetic field produced by a high frequency resonant coil. By selecting the duration of the laser pulse to be a small fraction of the period of the alternating magnetic field and controlling the timing of the pulse with respect to the phase of the alternating magnetic field, marks of varying length can be written. This technique desirably permits multiple beam recording, in which each beam is properly synchronized with a global resonating field. However, this resonant coil technique requires synchronizing the laser pulses with the coil current; and mark edges cannot be placed arbitrarily, which leads to special code constraints that reduce the code rate and linear density.
The Japanese Journal of Applied Physics, Vol. 26 (1987) at pp. 149-154 describes a technique whereby direct overwriting is achieved by use of a magnetic head which floats on an air-bearing slider, as in magnetic disk recording. The magnetic head produces a magnetic field that can be switched rapidly due to the low field volume. However, the circuitry for controlling the switching of the magnetic field is not disclosed. Moreover, a number of practical problems arise using this approach; e.g., aligning the pole piece with the focused beam, restriction of the technique to use with single-sided media, and the need for a clean environment to avoid head crashes. This reference cites, as prior art, a direct overwriting approach employing a fixed low-frequency magnetic head with wide spacing between the head and medium; and, because the head has to produce a large magnetic field to provide the field over a wide area, a large magnetic coil with a slow switching time must be used to apply an effective magnetic field to the medium.
Thus, prior approaches to direct overwriting have involved either resonating the magnetic field with the electric field of a capacitor or reducing the field volume considerably by bringing the magnet close to the active layer.
There is a need for an improved method and apparatus for direct overwriting on magneto-optic media and which, in combination, provides the following advantages: (1) a circuit arrangement that permits high speed switching of the magnetic field with less energy than required by previously proposed techniques; (2) permits patterns to be written without code constraints; (3) does not require operation of the coil of a magnetic head in a resonant mode; and (4) does not require a magnetic head that flies above the media.